This problem is typical of a bad electrical ground connection. I had a similar problem on my '83. The fix was to run a new ground wire jumper from instrument to instrument then to the large grounding block behind the instrument panel. It has been a bout five years now with no recurrence. Keep your original cluster if you can. It probably has original miles and sounds as though it is probably in better shape than any used one you will find.
Norma Jean is correct. The solution to your problem is to run another ground wire (brown in color is preferable) from the back of the instrument cluster (you will see a good place to ground the wire on the back of the cluster with one of the screws) to the grounding point on the steering column support where the other ground wires are attached. Make the new ground wire long enough to ease the removal of the instrument cluster in the future.
Thank you both!
That did the trick guys! All working well again. Now, on to replacement floor insulation for my 76 450SLC.....waiting for the kit from the SLShoppe in the U.K.
Good to hear. Sometimes it is the simple things like a ground that will frustrate you. Good job.
Thanks very much for reporting back. Most don't. Reporting ensures that it will help someone else some day.
If you ever have a need to remove your complete dash please let me know. I have removed and installed a number of W123 car dashes and several years ago I wrote a 3-4 page set of instructions on the steps to complete this task. Removing the dash is not complicated and utilizes common hand tools. The entire job can be completed (removal & Installation) in about 8 hours. If anyone would like the instructions just send me an email at BWhitmore2@earthlink.net and I will be happy to send them to you. Thanks.
Thanks. I now have two 123 instrument clusters available should anyone want one. :)
Good to know. You may want to post a For Sale ad on the PeachParts.com forum.
I am glad a fellow MBZ 300CD owner got the right help to keep the original instrument cluster. 1980's electronics can always be repaired. Something that I do and would recommend to others is that whatever you do to your car, make a note of it, record the miles, and the date. Make a spread sheet to keep things in date order and according to topic if you prefer. In this way, not only will you know your car's history, anyone else that owns the car afterwards can continue taking care of it properly and appreciate it more.
I have seen the grounding wire be a problem and some people get around this by connecting one side of the cluster to the other over the speedometer by a wire. Then have that go off into the car so there would be three points of contact with one wire. This insures that all parts of the cluster are grounded very well. the old factory wires that are built into the circuit boards break overtime so they can develop grounding issues. I had one on my car so I just placed a wire under the screw on each circuit board and that did the trick.